Tuberculosis (TB) is a chronic infectious disease caused by infection with Mycobacterium tuberculosis and other Mycobacterium species. It is a major disease in developing countries, as well as an increasing problem in developed areas of the world. More than 2 billion people are believed to be infected with TB bacilli, with about 9.2 million new cases of TB and 1.7 million deaths each year. 10% of those infected with TB bacilli will develop active TB, each person with active TB infecting an average of 10 to 15 others per year. While annual incidence rates have peaked globally, the number of deaths and cases is still rising due to population growth (World Health Organisation Tuberculosis Facts 2008).
Mycobacterium tuberculosis infects individuals through the respiratory route. Alveolar macrophages engulf the bacterium, but it is able to survive and proliferate by inhibiting phagosome fusion with acidic lysosomes. A complex immune response involving CD4+ and CD8+ T cells ensues, ultimately resulting in the formation of a granuloma. Central to the success of Mycobacterium tuberculosis as a pathogen is the fact that the isolated, but not eradicated, bacterium may persist for long periods, leaving an individual vulnerable to the later development of active TB.
Fewer than 5% of infected individuals develop active TB in the first years after infection. The granuloma can persist for decades and is believed to contain live Mycobacterium tuberculosis in a state of dormancy, deprived of oxygen and nutrients. However, recently it has been suggested that the majority of the bacteria in the dormancy state are located in non-macrophage cell types spread throughout the body (Locht et al, Expert Opin. Biol. Ther. 2007 7(11):1665-1677). The development of active TB occurs when the balance between the host's natural immunity and the pathogen changes, for example as a result of an immunosuppressive event (Anderson P Trends in Microbiology 2007 15(1):7-13; Ehlers S Infection 2009 37(2):87-95).
A dynamic hypothesis describing the balance between latent TB and active TB has also been proposed (Cardana P-J Inflammation & Allergy—Drug Targets 2006 6:27-39; Cardana P-J Infection 2009 37(2):80-86).
Although an infection may be asymptomatic for a considerable period of time, the active disease is most commonly manifested as an acute inflammation of the lungs, resulting in tiredness, weight loss, fever and a persistent cough. If untreated, serious complications and death typically result.
Tuberculosis can generally be controlled using extended antibiotic therapy, although such treatment is not sufficient to prevent the spread of the disease. Actively infected individuals may be largely asymptomatic, but contagious, for some time. In addition, although compliance with the treatment regimen is critical, patient behaviour is difficult to monitor. Some patients do not complete the course of treatment, which can lead to ineffective treatment and the development of drug resistance.
Multidrug-resistant TB (MDR-TB) is a form which fails to respond to first line medications. 5% of all TB cases are MDR-TB, with an estimated 490,000 new MDR-TB cases occurring each year. Extensively drug-resistant TB (XDR-TB) occurs when resistance to second line medications develops on top of MDR-TB. It is estimated that 40,000 new cases of the virtually untreatable XDR-TB arise annually (World Health Organisation Tuberculosis Facts 2008).
Even if a full course of antibiotic treatment is completed, infection with M. tuberculosis may not be eradicated from the infected individual and may remain as a latent infection that can be reactivated.
In order to control the spread of tuberculosis, an effective vaccination programme and accurate early diagnosis of the disease are of utmost importance.
Currently, vaccination with live bacteria is the most widely used method for inducing protective immunity. The most common Mycobacterium employed for this purpose is Bacillus Calmette-Guerin (BCG), an avirulent strain of M. bovis which was first developed over 60 years ago. However, the safety and efficacy of BCG is a source of controversy—while protecting against severe disease manifestation in children, BCG does not prevent the establishment of latent TB or the reactivation of pulmonary disease in adult life. Additionally, some countries, such as the United States, do not vaccinate the general public with this agent.
Almost all new generation TB vaccines which are currently in clinical development have been designed as pre-exposure vaccines. These include subunit vaccines, which have been particularly effective in boosting immunity induced by prior BCG vaccination, and advanced live mycobacterial vaccines which aim to replace BCG with more efficient and/or safer strains. Although these vaccines aim to improve resistance to infection, they are likely to be less effective as post-exposure or therapeutic vaccines in latent TB cases (Lin M Y et al Endocrine, Metabolic & Immune Disorders—Drug Targets 2008 8:15-29).
Example 2 of US20080269151 discloses the cloning, construction and expression of certain modified Rv3616c proteins, including: ΔTM-1, an Rv3616c polypeptide wherein residues 150 to 160 have been deleted (SEQ ID No: 22 of US20080269151); ΔTM-2, an Rv3616c polypeptide wherein residues 101 to 203 have been deleted (SEQ ID No: 24 of US20080269151); and a sequence wherein residues 150 to 160 of Rv3616c have been replaced by the antigen TbH9 (SEQ ID No: 60 of US20080269151).